PeakForce TUNA and Other AFM Modes for Electrical Properties Measurement

Typically, nanometer-scale electrical characterization is done by AFM-based conductivity measurements. Conductive AFM (CAFM) for high current ranges and tunnelling AFM (TUNA) for lower current range are the two categories of AFM. CAFM is a widely used technique, while TUNA represents the sensing module as well as the measuring technique for all current levels. Capabilities of TUNA are determined by the key elements, namely, current sensor, conductive AFM probe and base mode of AFM.

Different TUNA Modes

Contact TUNA

This mode uses a conductive tip and a current sensing module for its operation. Conventional applications of this technique include localizing and imaging the electrical defects in data storage and semiconductor devices, characterization of piezoelectric and ferroelectric materials, and conducting polymers. Contact mode cannot be used for topographic feedback as in samples of conductive polymers and loosely bound samples like nanowires that need low imaging forces in both the vertical or lateral directions.

Tapping Mode TUNA

In this mode, the AFM cantilever is oscillated at its fundamental flexural resonance mode, thereby the limitations of lateral forces during imaging as in the contact mode are eliminated. The vertical interaction force while imaging soft and delicate samples is lowered because of the high mechanical Q of the cantilever. Also, since tip contact is at a minimum, wear and tear of the tip is absent.

Torsional Resonance TUNA

In Torsional Resonance TUNA or TR TUNA, the AFM cantilevers oscillating in torsional modes produce images that help study a broad range of surface-tip interactions of soft delicate samples. A cantilever oscillating at first torsional resonance mode produces lateral forces that modify torsional resonant frequency, amplitude and/or the phase of the cantilever. The tip-sample contact changes with each oscillation; hence deviations might arise in measurements. Also limiting the amplitude to less than a few angstroms decreases the stability of operation.

PeakForce TUNA Module

The module is designed to have a bandwidth of 15kHz across a range of gains from 107 to 1010 V/A. This eliminates the need to change the module for different gain requirements, and a noise on cycle averaged current below 100fA.

Peak Force Tapping

In peak force tapping mode, the probe and the sample are in the tapping mode and are intermittently made to come in contact, thereby avoiding the lateral forces during imaging. The feedback loop controls the maximum force on the tip (peak force) for each cycle. The Peak Force tapping algorithm responds to the tip-sample force interaction with a modulation frequency (1 to 2kHz) lower than the cantilever’s resonant frequency.

PeakForce TUNA Results

Figure 2 shows the result of probe interaction with surface with top line representing Z-position, middle line showing force measured by the probe and bottom line representing detected current. The three measurements achieved from the graph are peak current (point C), cycle averaged current (from point A to E) and contact averaged current (point B to D).

Figure 2: Plots of Z position, force, and current as a function of time during one Peak Force Tapping cycle, with critical points including (B) jump-to-contact, (C) peak force, (D) adhesion labelled.

PeakForce TUNA Operation Modes

Imaging Mode

In this mode, an electrical probe is run over the sample in the peak force tapping mode and the feedback loop controls the peak force on tip, thereby wear of the tip and surface is minimised. The TUNA module then senses the current and presents the data in the form of topography image and mechanical properties maps.

I-V Spectroscopy Mode

This mode is used to measure the local current-voltage spectra by holding the tip in a fixed position while the sample moves up and down. The feedback loop maintains a constant deflection while the I-V curve is drawn.

Conclusions

Bruker's PeakForce TUNA technique is easy to use and proves to be the most capable method of force control mapping, especially for delicate samples. Bruker also provides an M-Braun’s glove box that protects the sample and the AFM measuring set up from external interferences. Figure 3 summarizes all the discussed above-discussed AFM techniques.

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